A typical ignition coil generally has a substantially T shape configured by a projecting shaft unit inserted into a plug hole, and a base end unit positioned above the plug hole. These units are integrated into one piece body.
According to a so-called pencil type ignition coil, the projecting shaft unit is provided with a primary coil having a cylindrical shape to house a center core, and a secondary coil disposed coaxially with the primary coil (Patent Document 1 and Patent Document 2).
In this configuration, however, a closed magnetic circuit structure is formed in correspondence with the cylindrical center core. Accordingly, coil output and power efficiency are limited.